Brassica napus (commonly referred to as canola or oilseed rape), which is grown in temperate climates of the northern and southern hemispheres, is an important cultivated oilseed crop species. While herbicide resistance has provided enhanced crop value, B. napus remains vulnerable to siliqua or pod shatter, resulting in significant seed loss, especially under adverse weather and harvest conditions. In crops with dry, dehiscent fruits, such as B. napus, the siliques or pods naturally release their seeds through a process called fruit dehiscence. When this process occurs prematurely, such as during adverse weather conditions (i.e., wind storm), seed recovery is reduced. This is especially problematic in crops where the oil from the seeds is desired. B. napus yield losses due to shatter fall within the range of 10%-25%, with increased losses observed as much as 50% when adverse climate conditions delay harvesting. Shatter can also result in the growth of volunteer plants or weeds.
Many plant species, including B. napus, disperse seed through the natural process of fruit dehiscence. In these species, siliques or pods are formed by two carpels that are separated by a thin replum. The dehiscence zone (DZ) is where the valve (fruit wall) margin connects to the replum. As the pod matures late in fruit development, the valve margin detaches from the replum, leading to seed dispersal. The DZ demarcates the precise location where the valves detach.
Several factors have been described to contribute to siliqua shatter resistance, including the morphology, anatomy and biochemistry of siliqua development and physiology, as well as environmental factors. Assessment of B. napus accessions for shatter resistance identified two resistant lines (Wen et al, 2008, Acta Agronomic Sinica 34: 163-166). Other studies of B. napus indicated limited genetic variation. Brassica rapa vars Yellow Sarson and Brown Sarson showed genetic variation in increased siliqua strength resulting in shatter resistance. Improved resistance to shatter was seen upon introgression of the trait from these Brassica types and B. juncea. 
Several genes have been identified with putative roles in shatter resistance, including genes involved in dehiscence zone differentiation and their regulatory genes (see review by Hossain et al., 2012, in Plant Breeding, Dr. Ibrokhim Abdurakhmonov (Ed), ISBN: 978-953-307-932-5, InTech at URL intechopen.com/books/plant-breeding/breeding-brassica-napus-for-shatter-resistance). WO 2012/084742 A1 discloses Brassica plants comprising mutant ALCATRAZ (ALC) genes, ALC nucleic acid sequences and proteins that confer increased pod shatter resistance and reduction or delay of seed shatter, as well as methods for generating and identifying the resistant plants and alleles. US 2012/0023603 A1 discloses plants that comprise at least two IND genes, whereby the plants comprise in their genome either two partial knock-out mutant IND alleles or two partial and two full knock-out mutant IND alleles, and confer reduced shattering while simultaneously maintaining an agronomically acceptable pod threshability. Many other genes with numerous putative functions are described in WO 2012/084742 and US 2012/0023603. It is evident from these disclosures that shatter is controlled by numerous and diverse genetic factors, which are additive and/or interrelated in their effect.
Early methods for evaluating shatter resistance were based on imprecise, subjective visual measurements and manual testing, using field observations, crude mechanical tests and anatomical tests (Hossain et al., 2012, supra). Subsequent mechanical testing methods were developed that demonstrated greater accuracy. Means of measuring the level of resistance to pod shatter tendency are known in the art and include, but are not limited to, the pendulum-based test, cantilever test, manual bending test, microfracture test (MFT), siliqua twisting, ‘Ripping’ method and Random Impact Test (RIT) (See Hossain, 2012, supra for review). U.S. Pat. No. 7,412,880 B2 describes a device and method for screening crop plants, including Brassica, for stalk strength, root lodging, and/or other wind damage resistance traits by selectively applying wind forces to stands of plants in an agricultural environment. Current methods employed to reduce shattering include windrowing (swathing) and spraying desiccants, resulting in increased costs and less flexible farming practices (see Hossain, 2012, supra, for review)
What is needed in the art and industry is a means to identify genes or germplasm conferring resistance to shatter using molecular markers. These markers can then be used to tag the favorable alleles of these genes in segregating populations and then employed to make selection for resistance more effective, and to combine several resistance sources in a single genotype that has a high level of shattering resistance. The present invention provides these and other advantages.